A structure has been known which is configured to reduce wave sounds of fuel by suppressing waving of the fuel using a wave-eliminating plate fixed to an inside of a resinous fuel tank installed in an automobile or the like. A technique for realizing this structure is described in Patent Literature 1, for example. In this technique, a wave-eliminating plate (an inner skeleton in Patent Literature 1) is directly welded and fixed at multiple positions to an inner wall of a fuel tank.